Roller-Compacted Concrete Delivers Rapid, Efficient Dam Rebuild
The rebuilding of Taum Sauk Upper Reservoir is a project of many superlatives. The upper bowl of the 440-MW pumped-storage system sits on top of Missouri’s highest peak, 1,590-ft Proffit Mountain. It is believed to be North America’s largest roller-compacted concrete dam. When the original earth-and-rockfill dike was over- topped and failed in December 2005, Missouri Gov. Matt Blunt (R) called the damage done by the 4,365 acre-ft of water it released “the worst man-made disaster in the history of Missouri.” And the $10-million civil penalty imposed on St. Louis-based utility AmerenUE by the Federal Energy Regulatory Commission for the failure was the largest FERC penalty ever in a hydroelectric case.
Today, the reservoir is nearly complete, and preparations are under way to start commercial operation. Roller-compacted concrete (RCC) placement was completed in November, and the new dam was topped out. Engineer-of-record Paul C. Rizzo Associates Inc. will begin a refill-testing program required by FERC in the first week of February and expects to complete it by March 1. “We could start commercial operation April 1,” says Carl M. Rizzo, vice president of construction management services.
A pumped-storage system takes advantage of differential electricity rates to store power generated when rates are low, then sells it when rates are higher (see graphic). The Taum Sauk system pumps water overnight from the lower reservoir, which is at an elevation of 730 ft, through the 6,300-ft-long penstock and 26-ft-dia vertical shaft to the upper reservoir at 1,500 ft. The pumps are later reversed and draw the water into the shaft and penstock to the powerhouse at 767 ft, where it drives two 220-MW hydroturbines and is discharged to the lower reservoir.
The original 4,365-acre-ft Taum Sauk Upper Reservoir in Reynolds County breached shortly before dawn on Dec. 14, 2005, emptying out in a 20-ft-high wave down the side of Proffit Mountain, through a state park and the system’s lower reservoir and into the Black River. No deaths resulted, but the flood devastated a popular recreation area, aroused fears for safety and took the Taum Sauk system off-line.
AmerenUE took full responsibility for the failure after its preliminary investigation pointed to an instrumentation failure. The utility retained Paul Rizzo, head of the Monroeville, Pa., firm that bears his name, to investigate. Rizzo’s April 2006 report cited stability failure stemming from poor-quality material, which deviated from the original design specifications, and poor construction practices as root causes of the failure. Construction practices were consistent with early 60s standards but not with construction practices used today. Contributing factors of human error and inadequate instrumentation led to the overfilling of the reservoir and the overtopping of its dam.
Settlements
In a series of settlements with state and federal authorities, AmerenUE cleared the way to rebuild the upper reservoir. But the effort was costly: In addition to the $10-million FERC penalty, AmerenUE settled for $180 million in cash and property with the state. In November 2007, AmerenUE announced the selection of the team that would rebuild the upper reservoir: Rizzo Associates was named the engineer-of-record and project manager, and Ozark Constructors LLC, a 50-50 joint venture of dam-builder ASI Constructors Inc., Pueblo West, Colo., and St. Louis-based Fred Weber Inc., was named construction contractor. Rizzo already had designed the replacement reservoir, so work was set to begin immediately.
The reservoir “bowl” occupies 40 acres on the leveled mountaintop. In addition, 35 acres were cleared and...
...leveled for laydown and to fit offices, warehouses, mechanic shops, general storage, two RCC plants, a conventional concrete batch plant, parking for 600 cars and a 750,000-gallon reservoir for production water on the mountain outside the bowl.
All the aggregate for the RCC as well as the conventional concrete was produced by crushing the rocks from the old dam. Because it was an earth-and-rockfill dam, some of the material had a parasitic coating of loose dirt. “It’s not uncommon for dirtier material to be used in RCC,” says Lee Schermerhorn, Ozark construction manager representing ASI Constructors in the joint venture. The new dam required 3 million cu yd of crushed rock, and by the end of the project they were scraping the bottom of the barrel and telling people to shake out their boots and pockets before going home, he jokes. “We used every bit of it,” Rizzo agrees. “We had very little waste piled at the end.”
Fred Weber Inc. began setting up a crushing plant in March 2007, and production crushing began in September. Ozark Constructors began placing roller-compacted concrete under a $283-million cost-plus-fixed-fee contract with AmerenUE in late October.
The new dam uses 2.8 million cu yd of RCC and 300,000 cu yd of conventional facing concrete. “It’s the largest RCC dam in North America,” says Carl Rizzo. It is 1.26 miles long, 120 ft tall and 150 ft wide at the base. The reservoir’s storage volume is the same as the original’s. RCC has been used in much larger dams elsewhere. China’s Long Tan Dam is the world’s largest RCC dam, according to Malcolm Dunstan & Associates, a U.K.-based engineer that specializes in design of RCC structures. Long Tan stands 710 ft tall and contains 6.474 million cu yd of RCC. Completion was scheduled in 2009.
The biggest challenge at Taum Sauk was not the dam’s size; it was sequencing the work because of the dam’s configuration, says Rizzo. For a conventional dam, the RCC system is set up to deliver the concrete to a fixed point, with conveyor extensions added or removed as the leading edge moves. But the Taum Sauk Dam consists of nine monoliths with no abutments. “The excavation drove the placement of RCC,” says Rizzo. The work was done in all directions around the RCC plants, which were set up inside the bowl. The frequent change of delivery points meant “you couldn’t do it continuously,” he says. Instead of stationary conveyors, the contractor used mobile telescoping conveyors to deliver concrete.
The foundations also presented difficulties. Clay seams run through the site with geologic anomalies, says Rizzo. “When this dam was built in 1964, it was dumped rockfill,” he says. The builder shaved off the mountaintop and placed the excavated material in a kidney-shaped configuration to form the reservoir. “They didn’t consider the foundations at all.”
Redesigning the reservoir after its catastrophic failure, Rizzo Associates took extra precautions. “When we did the subsurface investigation we realized that we had some clay seams throughout the foundation,” including a clay seam “we called a geologic anomaly—a shear zone that ran directly across the reservoir from east to west.” The RCC could not be placed on the clay because it couldn’t support the structure so it had to be removed. “We had to take it down some 40-50 ft below the designed foundation elevation.”
Serious Kink
The discovery threw a serious kink in the schedule. “When we found the geologic anomalies, it was like hitting a wall at 90 mph,” says Roger Gagliano, Ozark construction manager representing Fred Weber Inc. in the joint venture. “We pushed the labor to beat the schedule.” Labor availability also was a problem early in the project. Holcim (U.S.) Inc. broke ground for a 4-million-tonne-per-year cement plant in nearby Ste. Genevieve County, Mo., in 2006, and work was still in full swing in summer 2008. “It was a struggle to get enough people because the Holcim plant was in construction,” says Mark Denton, Ozark project manager.
Some excavations went as deep as 60 ft, while others were superficial, but all had to be brought back up with “leveling” concrete before RCC could be placed. The extra excavation cost the project 70 to 80 days, Rizzo says. Even with this delay, the project’s cost and schedule have not slipped much. Weather has been the main cause of delay. “It’s very unusual to place RCC in the wintertime,” he adds.
“RCC placements are very temperature-sensitive,” agrees Ozark’s Schermerhorn. “If it was too hot, we couldn’t place as much because we had certain parameters we had to stick within, and it was the same with rain or cold.” Ozark operated two 10-hour shifts six days per week in spring and fall, cutting back to two 8-hour shifts in summer. It did little RCC placement at all from early December to early April. When the last RCC was placed in November 2009, placement had occurred in only 18 of the project’s 24 months.
Roller-compacted concrete is a stiff, zero-slump concrete mixture with the consistency of damp gravel, according to the Portland Cement Association, Skokie, Ill. PCA says the durable paving material was developed in Canada to carry heavy loads, such as loaded logging trucks in remote areas. The U.S. Army Corps of Engineers first used RCC for a gravity dam in 1982 at Willow Creek Dam in Oregon. Since then it has been increasingly used for dam construction.
“[Conventional] concrete is really not a good thing to build a dam out of,” says Schermerhorn. “[RCC] is a low-heat mix design. It won’t crack as much. If you built the whole dam out of conventional concrete, it would crack because the heat buildup would be too much.”
Dam builders using conventional concrete have had to work around the drawbacks of cement’s chemistry. Schermerhorn points out, “You build up high heat in the core of mass concrete, so it expands. As it cures, the...
...temperatures go up, reach a peak point and then start coming back down.” The resulting thermal stress produces cracks.
In the 1980s, engineers tried reducing the cement and water in the mix to reduce the thermal stress. The resulting mix could be placed in a mass by dump trucks, spread by bulldozers, then compacted with smooth-drum vibratory rollers. Once they figured out the process, it proved to be an efficient way to build a gravity dam. “They found out that 100,000 cu yd of RCC” could substitute for “1 million cu yd of dirt,” says Schermerhorn. If there is “a good, solid foundation on rock, RCC is a lot more economical to build,” he adds. RCC was the only material considered for the several design alternatives for Taum Sauk, Rizzo says. “It was the most cost-effective and expedient way to rebuild the reservoir,” he adds.
Two RCC batch plants inside the bowl and one outside produced up to 14,000 cu yd of RCC per day, depending on weather and the number of shifts that the weather allowed, says Schermerhorn. Each plant had a pair of mixers. One was top-loaded with aggregate, cement and water, while the other dumped 7.7 cu yd of RCC onto a belt going to the placement site. The cycles alternated at 30-second intervals, producing a constant flow of RCC. Each plant inside the bowl supplied RCC to one of the dam’s nine monoliths. A rock-crushing plant also was located in the bowl. The third RCC plant, outside the bowl, fed another site, usually in the foundation, says Schermerhorn. Its production was delivered by dump truck, telebelt or mobile telescopic conveyor, rather than the belt system used inside the bowl.
Bump Up Production
A fourth roller-compacted-concrete batch plant was added in late summer 2009 to bump up production as pressure grew to complete RCC placement before winter. A structural concrete batch plant also was located outside the bowl to produce concrete for the upstream and downstream faces of the structure.
One tragedy marred the otherwise successful project. The project’s safety record was good for the first 10 months of the work, and in August 2008 the team celebrated the first million work-hours without a lost-time accident. But one accident broke the streak. Nine months later, in May 2009, Connie Munton, a laborer, was struck and killed by a dump truck working on a night time RCC placement.
The U.S. Occupational Safety and Health Agency investigated but closed the investigation in October without issuing a citation. “We found no basis for a citation being issued to Ozark Constructors,” says Bill McDonald, OSHA’s St. Louis area director. As the project comes to a close, more than 3 million work-hours have been logged.
The old reservoir’s failure manifestly had devastating consequences, and Rizzo Associates designed the new one to prevent the possibility of overtopping again. But another known threat is seismicity. The New Madrid Seismic Zone, which experienced a series of quakes up to 8.3 on the moment magnitude scale in 1812, is just 74 miles away. The new reservoir should be able to withstand a design-basis ground motion of 7.7 from that source, says Rizzo. As far as humanly possible, the new Taum Sauk Upper Reservoir is designed and built to justify AmerenUE President and CEO Thomas Voss’ 2007 statement, “After much analysis, we are now confident that this plant can be returned to service and operated safely to restore a critical source of reliable power to our customers.”
